Optical probe containing oxygen, temperature, and pressure sensors and monitoring and control systems containing the same

ABSTRACT

A probe for measuring oxygen, temperature, and pressure having a housing, made of a thermally conductive material; an oxygen sensor within the housing, a temperature sensor disposed within the housing adjacent to the thermally conductive material, comprising a fiber Bragg grating, a pressure sensor disposed within the housing, comprising a fiber Bragg grating.

This application claims benefit of the filing date of U.S. ProvisionalApplication No. 61/406,050, filed Oct. 22, 2010, the entire contents ofwhich is incorporated herein by reference.

BACKGROUND

1. Field

Disclosed herein is an optical probe containing integrated oxygen,temperature, and pressure sensors. The probe is particularly suitablefor determining the concentration of oxygen, particularly the partialpressure of oxygen, in an enclosed space, such as a fuel tank, cargohold, passenger compartment, or other space in a vehicle, such as anaircraft, ship, boat, land vehicle, or other military, commercial, oraerospace vessel.

2. Description of Related Art

Since 1959, a number of aircraft fuel tanks have unexpectedly exploded.Typically, the explosions occurred when an unknown ignition sourceignited the fuel/vapor mixture in the fuel tank. Fuel/vapor mixtures arecreated during consumption of fuel within the fuel tank by engines ofthe aircraft. The consumed fuel leaves a space within the tank whichgenerally fills with atmospheric air containing oxygen. The presence ofboth a flammable gas and the fuel/vapor mixture within the space createsthe potential for an explosion within the fuel tank upon ignition. Theindustry has responded with various methods and apparatuses, asdiscussed in Air Safety Week, Vol. 15 No. 16, Apr. 16, 2001, “FatalExplosion Highlights Hazard of Flammable Vapors in Fuel Tanks.” Inaddition, recent combat experience with fixed wing and rotary aircraftsuggests that fuel tanks are susceptible to penetration by hostile fire,and in particular by shrapnel and small arms fire, which creates both aningress for oxygen and a potential ignition source. Moreover, iflightning penetrates the wing or fuel tank of an aircraft as the resultof a lightning strike, it can be a significant source of ignition,particularly in aircraft constructed using composite materials.

One method which reduces fuel/vapor combustion includes the eliminationof combustible gases from the fuel tank. This method fills space withina fuel tank with an inert gas. The presence of the inert gas within thefuel tank deprives the fuel/vapor mixture of a flammable gas necessaryfor combustion. Nonetheless, the need to continuously fill the fuel tankwith an inert gas and the attendant high costs associated therewith donot make this an attractive alternative for aircraft manufacturers.

A more efficient method in accordance with the prior art includesflooding the tank with inert gas when oxygen levels become high. Thismethod requires continually measuring oxygen levels in a fuel tank.However, in order to accurately determine the oxygen concentration,either the temperature level of the oxygen sensor must be kept constant,or the temperature of the sensor must be measured in real time and takeninto account in calculating the oxygen partial pressure from the sensorsignal. Similar concerns arise with regard to the pressure in thevicinity of the oxygen sensor. However, temperatures and pressureswithin an enclosed space, such as fuel tanks in vehicles, can fluctuateover time depending on the outside temperature. In addition, spatialfluctuations in temperature and pressure within the fuel tank can occur.Moreover, these fluctuations can impact the ability of the monitoringand control electronics to accurately determine the oxygen level fromthe data obtained from the oxygen sensor.

Prior art attempts to address similar issues involved keeping thetemperature of a gas sensor constant include heating the gas sensor withelectric resistance heaters when the temperature is low. However, thesemethods are not suitable for use in fuel tanks, as electrical currentapplied to the electrical resistance heaters may potentially ignite thefuel/vapor mixture within the tank, again making this an unattractiveoption for aircraft manufacturers. Other attempts to provide fuel tankinerting systems and sensors include those described in U.S. Pat. Nos.6,634,598; 6,904,930; 6,925,852; and 7,231,809. An oxygen probecontaining a temperature sensor has also been developed, but the absenceof an integrated pressure sensor requires that the system use pressureinformation from the aircraft pilot tubes. Such a system providespressure information that is representative of the pressure in the fueltank, but is based on the pressure outside of the aircraft.Alternatively, a pressure sensor system built into the fuel tank at afixed position can provide erroneous information when the aircraft isnot level. As a result, the pressure measurement from such systems isnot accurate information about the ullage pressure in the vicinity ofthe probe, where the oxygen level is actually being measured.

Therefore, a need exists for a method and apparatus which providesaccurate information about the temperature and pressure environment inthe vicinity of the oxygen sensor, and that does not require potentialsources of ignition, particularly in a vehicle fuel tank.

In addition; some vehicles, such as commercial and military aircraft,contain areas, such as cargo holds, passenger compartments, and thelike, that may, expectedly or unexpectedly, contain materials capable ofsupporting combustion. Such areas can be equipped with fire suppressionsystems, which often are manually operated from the flight deck when anindication of a fire is received, generally from an increase intemperature in the cargo hold. However, there is typically no access tothe cargo hold from the flight deck, and even if the fire suppressionsystem is effective, temperature in the cargo hold may remain elevatedfor a considerable period of time. As a result, with such a system thereis limited ability of the flight crew to determine in the short termwhether the fire suppression system has been successfully deployed andhas been effective in controlling the fire. Accordingly, there remains aneed for a monitoring system that can allow a more rapid and accuratedetermination of whether fire suppression and control systems have beeneffective.

Fire suppression in passenger compartments provides a particularchallenge, requiring precise control of the type and amount of firesuppression gas introduced, so as to decrease oxygen available forcombustion while maintaining sufficient oxygen for life support of thepassengers. Accurate monitoring of oxygen concentration in such a spaceis essential.

One technique that might be suggested as suitable for fuel tank inertingor monitoring is the On Board Inert Gas Generating System (OBIGGSsystem). This system processes pressurized air through hollow fibermembranes to obtain a nitrogen enriched air, which can be used as aninerting gas. However, the implementation of this system is not optimal,because of the lack of an appropriate sensing/control system. As aresult, attempts have been made to operate OBIGGS equipped aircraft withthe system constantly operational (i.e., continuously supplying nitrogento the ullage of the aircraft fuel tanks). Such an operation, however,incurs a significant fuel penalty. Accordingly, there remains a need inthe art for a sensing/control system that allows an inerting system suchas OBIGGS to be operated when necessary (i.e., when the oxygen partialpressure in the ullage of the fuel tank reaches a predetermined value)and to be idled when operation is not necessary, thereby increasing fueleconomy. Such an idled mode includes heating of the system to preventfreezing of moisture in the system.

Accordingly, there remains a need in the art for a probe that providesaccurate, localized information about oxygen concentration, pressure,and temperature, and that is capable of operating under the stringentenvironmental conditions found in, e.g., an aviation fuel tank. Theseconditions include operation under widely varying temperatures,operation under low temperatures, operation while exposed to thecomponents of fuels, and particularly while exposed to the hydrocarbonsin various aviation fuels, such as jet fuels, and operation undervibration.

SUMMARY

In one embodiment is disclosed a probe for measuring oxygen,temperature, and pressure in a space to be monitored, comprising:

a housing, comprising a thermally conductive material;

an oxygen sensor disposed within the housing, comprising:

-   -   a first end having coated thereon a coating which fluoresces at        a fluorescent frequency when exposed to light having an        excitation frequency in the absence of associated oxygen, and        which undergoes a dampening of said fluorescence in the presence        of associated oxygen; and    -   a second end operatively connected to an optical fiber that        extends through the housing;

wherein the first end extends through the housing and is adapted to beexposed to the space to be monitored;

a temperature sensor disposed within the housing adjacent to thethermally conductive material, comprising a fiber Bragg grating, whereinthe temperature sensor does not extend through the housing and is notexposed to the space to be monitored;

a pressure sensor disposed within the housing, comprising a fiber Bragggrating having a first end which extends through the housing and isadapted to be exposed to the space to be monitored.

In another embodiment is disclosed a system for monitoring the level ofoxygen in a space to be monitored, comprising:

the probe described herein; and

an analyzer for calculating oxygen partial pressure based upon thefluorescence damping, temperature, and pressure data provided by theprobe.

In another embodiment is disclosed a system for controlling theconcentration of oxygen in a space, comprising:

the system for monitoring the level of oxygen in the space, as describedherein; and

a controller for introducing an inert gas into the space when the levelof oxygen in the space reaches a predetermined level.

In another embodiment is disclosed a vehicle comprising the system forcontrolling the concentration of oxygen in a space as disclosed herein.

In another embodiment is disclosed a method for monitoring the level ofoxygen in a space equipped with a probe as described herein, comprising:

obtaining fluorescence damping data from the oxygen sensor;

obtaining temperature data from the relative Bragg wavelength shift ofthe temperature sensor;

obtaining pressure data from relative Bragg wavelength shift of thepressure sensor taking into account the temperature data from thetemperature sensor;

determining the partial pressure of oxygen from said fluorescencedamping data, said temperature data, and said pressure data.

In another embodiment is disclosed herein a method for controlling thelevel of oxygen in a space, comprising:

monitoring the level of oxygen in the space as disclosed herein;

controlling the introduction of an inert gas into the space byintroducing said inert gas into the space when the level of oxygenreaches a predetermined level.

Because each of the sensors is located at the probe tip, data on oxygenconcentration, temperature, and pressure are obtained at that location,providing a more accurate, real time, in situ determination andcalculation of oxygen concentration, and therefore better monitoring andcontrol of the level of oxygen in the space being monitored. Moreover,because no sample removal is required and data acquisition occurs withinthe space being monitored, accuracy is also increased. For example,embodiments of the probe and system described herein provide a sensordynamic range of oxygen concentration ranging from 0% (total inertness)to 25% (in excess of air), with a resolution of 0.1% and an accuracy of0.5% O₂, and a sensor response time of a few seconds.

The use of fiber optics, rather than electronics, to gather and transmitdata decreases the risk of an unanticipated ignition source as comparedto electronic systems, because it is not the source of electrical orelectrostatic sparks, does not produce a current, and is not otherwisean ignition source. Moreover, the probe and systems described herein isnot affected chemically, physically, or functionally by exposure toliquid or vaporized fuel, which decreases the risk of adverse affects bycontact with hydrocarbons (e.g. adverse interactions between aircraftfuel and galvanic cell membranes), and eliminates the need for heatingresulting from the use of zirconium electrochemical cells, as well aseliminates any consumption of oxygen to operate the system. Moreover,the systems described herein are expected to require less frequentcalibration, and to be less susceptible to interferences (such as EMIinterference) than existing technologies. Embodiments of the probedescribed herein can be multiplexed to a single control unit, allowingmultiple locations in a space to be monitored simultaneously.

Embodiments of the probe described herein are resistant to hydrocarbonexposure, resistant to the effects of thermal shock and pressure changesexperience, e.g., during flight. For example, embodiments of the probeand system described herein can operate over a temperature range of −50to +80° C., and over a pressure range of ambient to +2 psi. The probedescribed herein allows the vehicle operator to confirm that inerting orfire suppression is occurring as needed and according to specifications,to determine the inertness of the space being monitored, and allows fora closed loop control of oxygen levels in the space being monitored.Moreover, the system is small, light weight, and compatible withautomatic control systems onboard the vehicle.

BRIEF DESCRIPTION OF DRAWINGS

Illustrative embodiments disclosed herein can be more clearly understoodby reference to the following drawings, wherein the same referencenumerals indicate the same structure.

FIG. 1 is a schematic view of an integrated oxygen probe having oxygen,temperature, and pressure sensors according to an embodiment disclosedherein.

FIG. 2 is a perspective view of an embodiment of an oxygen probe tipsuitable for use in the oxygen sensor of FIG. 1

FIG. 3A is a front plan view of the oxygen probe tip of FIG. 2. FIG. 3Bis a sectional view along section line A-A of FIG. 3A.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

As used herein, the term “about” in connection with a numerical value orrange of numerical values denotes somewhat above and somewhat below thestated numerical value, to a maximum deviation of ±10%.

A particular embodiment of an integrated oxygen, temperature, andpressure probe is shown schematically in FIG. 1. Probe tip 1 is disposedat one end of probe tube 3. At the other end of probe tube 3 is disposedprobe cover 2, adjacent to heat shrink tube 10. The probe tip 1 containsan oxygen sensor, a temperature sensor, and a pressure sensor, allconnected to optical fibers (4, 5, and 6) (e.g., 1000-1100 micron fiberis generally suitable, however other fiber sizes can be used) enclosedwithin a polymeric tube (e.g., PTFE tube). In the illustratedembodiment, each optical fiber is connected to an optical connector (8A,8B, and 8C) suitable for connection to an optoelectronic monitoringand/or control system (not shown). FIG. 1 provides representativedimensions with respect to distances, radii of curvature, and angularmeasurements for a particular exemplary embodiment, and may be variedsubstantially as specific circumstances require or permit. For example,R, representing a radius of curvature of the probe, may generally begreater than or equal to 0, more particularly greater than or equal to1.3 inches, and is dependent to some degree on the properties andthickness of the optical fibers used in the probe. In some applications,no curvature in the probe will be necessary or desirable, so that therewill be no radius of curvature. Similarly, angle γ will also depend uponthe thickness and properties of the fiber, and may range from 0° to 75°,more particularly from 0° to 60°, even more particularly from 0° to 45°.The length L can also be substantially variable, but is generallygreater than or equal to 3 inches, more particularly greater than orequal to 5 inches. Factors that may influence the various dimensions andgeometry of the system include the particular platform in which thesystem is installed, the particular mounting used, clearances in thevicinity of the mounting, etc.

FIG. 2 is a perspective view of an embodiment of probe tip 1, showingdistal end 102, which is adapted to connect to one end of probe tube 3,and proximal end 104, which is exposed to the environment of the spaceto be monitored. In the embodiment shown, proximal end 104 contains aflat portion 106 and a beveled portion 108, although this geometry isexemplary and other geometries are possible. Flat portion 106 containspressure sensor 110 and temperature sensor 116 (shown as a dotted linein FIG. 2). The proximal end of pressure sensor 110 is, in thisembodiment, exposed to the environment of the space to be monitored,while temperature sensor 116 is separated from the environment of thespace to be monitored by a thin layer of thermally conductive material,desirably the metal forming the flat portion 106 of the probe tip 1.Beveled portion 108 contains oxygen sensor 112, the proximal end ofwhich is also exposed to the environment of the space to be monitored.Between proximal end 104 and distal end 102 is probe tip body 114,which, like distal end 102 is desirably hollow, allowing sufficientspace for the fibers connected to pressure sensor 110, temperaturesensor 116 and oxygen sensor 112 to pass through the probe tip 1 to theprobe tube 3, and to provide sufficient rigidity. The probe tube 3 candesirably be filled with vibration dampening material, such as silicone,indicated by 3 a in FIG. 1, to restrict fiber movement and/or dampenvibrations experienced by the system during operation.

FIG. 3A is a front plan view of proximal end 104 of probe tip 1, showingthe arrangement of flat portion 106 and beveled portion 108, as well asthe arrangement of pressure sensor 110, temperature sensor 116 (shown indotted line) and oxygen sensor 112.

FIG. 3B is a sectional view along section line A-A in FIG. 3A.Temperature sensor 116 is visible in this view, and is separated fromthe environment to be monitored by a thin piece of thermally conductivematerial 118. The angle of bevel a can be substantially variable, butgenerally ranges from about 30° to about 60°, more particularly about45°.

The oxygen sensor may desirably be of the fluorescence damping typedescribed in U.S. Pat. Nos. 6,634,598; 6,904,930; 6,925,852; 7,231,809,7,740,904, and in U.S. Patent Application Publication Nos. 2008/0199360and 2009/0028756, the entire contents of each of which are incorporatedherein by reference.

The temperature sensor may be of a fiber Bragg type, wherein the Braggwavelength is sensitive to temperature. Without wishing to be bound bytheory, it is believed that a change in temperature in the space beingmonitored results in a shift in the Bragg wavelength Δλ_(B)/λ_(B),according to a relationship of the form:

$\left\lbrack \frac{\Delta\;\lambda_{B}}{\lambda_{B}} \right\rbrack = {{C_{S}ɛ} + {C_{T\;}\Delta\; T}}$wherein ε is an applied strain, C_(S) is the coefficient of strain,C_(T) is the coefficient of temperature, and ΔT is the temperaturedifference. By maintaining the temperature sensor behind a layer ofthermally conductive material, the contribution of strain to the shiftin Bragg wavelength is reduced or eliminated, and the shift is thus theresult of the change in temperature in the space to be monitored, atleast a portion of which is seen by the fiber Bragg sensor through thethermal conduction of the layer of thermally conductive material.

The pressure sensor may also be of a fiber Bragg type, wherein the tipof the fiber Bragg grating is directly exposed to the space to bemonitored. Without wishing to be bound by theory, it is believed thatthe resulting shift in Bragg wavelength will be the result of thecontribution of the strain experienced by the pressure sensor as theresult of the change in pressure, and of the contribution from thechange in temperature. Since the temperature contribution is known fromthe temperature sensor, the strain contribution can be determined via asuitable algorithm, and the resulting pressure change determined.

Suitable fiber Bragg grating transceiver systems suitable for use withthe fiber Bragg grating temperature and pressure sensors described aboveinclude those available from Redondo Optics, Inc. (FBG-Transceiver™).Pressure, temperature, and oxygen level data obtained from the probedescribed above may be sent to an analysis system, such as amultichannel interrogation system. One example of a suitable system isthe FOxSense™ multichannel interrogation system available from RedondoOptics, Inc. The data is analyzed using a suitable algorithm, such asone using the Stern-Volmer relationship, to determine the partialpressure of oxygen in the space being monitored, such as the ullage of afuel tank, cargo hold, passenger compartment, etc. This information can,in turn, be used to control and monitor an inerting system of the typedescribed in U.S. Pat. Nos. 6,634,598; 6,904,930; 6,925,852; and7,231,809, an OBIGGS system, a halon fire suppression system, anitrogen/water mist passenger compartment fire suppression system, andthe like, on an aircraft, waterborne vessel, tank or armored vehicle,etc.

Desirably, if the probe and the monitoring and control systems describedherein are operated to provide inerting of, e.g., a vehicle fuel tank,the oxygen concentration therein is maintained at a level below about 9%by volume (for military aircraft) and below about 11-12% by volume (forcommercial aircraft) at atmospheric pressure.

What is claimed is:
 1. A probe for measuring oxygen, temperature, andpressure in a space to be monitored, comprising: a housing, comprising athermally conductive material; an oxygen sensor disposed within thehousing, comprising: a first end having coated thereon a coating whichfluoresces at a fluorescent frequency when exposed to light having anexcitation frequency in the absence of associated oxygen, and whichundergoes a dampening of said fluorescence in the presence of associatedoxygen; and a second end operatively connected to an optical fiber thatextends through the housing; wherein the first end extends through thehousing and is adapted to be exposed to the space to be monitored; atemperature sensor disposed within the housing adjacent to the thermallyconductive material, comprising a fiber Bragg grating, wherein thetemperature sensor does not extend through the housing and is notexposed to the space to be monitored; a pressure sensor disposed withinthe housing, comprising a fiber Bragg grating having a first end whichextends through the housing and is adapted to be exposed to the space tobe monitored.
 2. A system for monitoring the level of oxygen in a spaceto be monitored, comprising: the probe according to claim 1; and ananalyzer for calculating oxygen partial pressure based upon thefluorescence damping, temperature, and pressure data provided by theprobe.
 3. A system for controlling the concentration of oxygen in aspace, comprising: the system for monitoring the level of oxygen in thespace, according to claim 2; and a controller for introducing an inertgas into the space when the level of oxygen in the space reaches apredetermined level.
 4. A vehicle comprising the system for controllingthe concentration of oxygen in a space according to claim
 3. 5. A methodfor monitoring the level of oxygen in a space equipped with a probeaccording to claim 1, comprising: obtaining fluorescence damping datafrom the oxygen sensor; obtaining temperature data from the relativeBragg wavelength shift of the temperature sensor; obtaining pressuredata from relative Bragg wavelength shift of the pressure sensor takinginto account the temperature data from the temperature sensor;determining the partial pressure of oxygen from said fluorescencedamping data, said temperature data, and said pressure data.
 6. A methodfor controlling the level of oxygen in a space, comprising: monitoringthe level of oxygen in the space according to claim 5; controlling theintroduction of an inert gas into the space by introducing said inertgas into the space when the level of oxygen reaches a predeterminedlevel.